The hooded seal, also called the bladdernose seal (Cystophora cristata), is a large type of seal found only in the central and western parts of the North Atlantic Ocean. These seals live from Svalbard in the east to the Gulf of St. Lawrence in the west. They have silver-grey or white fur with black spots of different sizes covering most of their bodies. Baby hooded seals, called "blue-backs," have blue-grey fur on their backs and whitish fur on their bellies. They lose this coat when they are 14 months old. Hooded seals are the only species in the group called Cystophora.
Naming
The scientific name Cystophora comes from the Greek words for "bladder-bearer," which describe a unique feature of the species: a special, inflatable bladder-like structure on the head of adult males. This bladder is located between the eyes and hangs over the upper lip when it is not inflated. Additionally, the hooded seal can inflate a large, balloon-like sac from one of its nostrils. This happens by closing one nostril and filling a membrane, which then sticks out from the other nostril.
Size
Adult males typically measure about 2.6 meters (8 feet 6 inches) in length, can reach up to 3.5 meters, and weigh between 300–410 kilograms (660–900 pounds). Males and females have different physical traits from birth, with females being smaller, averaging 2.03 meters (6 feet 8 inches) in length and weighing between 145–300 kilograms (320–661 pounds). Their body color is silver, with dark, irregular spots scattered across the skin. The head is darker than the rest of the body and lacks any spots.
Distribution and habitat
Hooded seals mainly live on floating ice and in deep water in the Arctic Ocean and North Atlantic. Some move to warmer areas during the year, but they survive best in cold climates. They are found in four main regions with floating ice: near Jan Mayen Island (northeast of Iceland); off Labrador and northeastern Newfoundland; the Gulf of St. Lawrence; and the Davis Strait (off midwestern Greenland). Males tend to stay near areas with complex ocean floor, such as Baffin Bay, Davis Strait, and the Flemish Cap, while females usually live in shallow areas, like the Labrador Shelf. Hooded seals are highly migratory and often travel long distances, reaching as far west as Alaska and as far south as the Canary Islands and Guadeloupe. Before the mid-1990s, hooded seals were rarely seen in Maine and the eastern Atlantic, but sightings increased after that time. Between 1997 and 1999, 84 hooded seals were seen in the Gulf of Maine, with one each in France and Portugal. From 1996 to 2006, five hooded seals were spotted or stranded near the Spanish coasts in the Mediterranean Sea. Scientists have not yet explained why hooded seals are being seen more often or in new areas.
Diet
The hooded seal's diet mainly includes amphipods (crustaceans), krill, and several types of fish such as Atlantic argentine, capelin, Greenland halibut, flounder, polar cod, herring, and redfish. They also eat octopus, squid, sea stars, shrimp, other crustaceans, and mussels. Compared to other seals, hooded seals eat three times as much redfish as other species. Capelin makes up about the same amount in their diet as in other similar seals. Capelin is a more common food choice during winter. Their diet is rich in lipids and fatty acids.
Behavior
Hooded seals usually swim in deep water, from 100 to 600 meters (330 to 1,970 feet), and can hold their breath for 5 to 25 minutes during dives. Some dives may reach depths of over 1,016 meters (3,333 feet) and last more than 52 minutes. Hooded seals spend about 90% of their time underwater during both day and night. Dives during the day are generally deeper and longer than those at night. In winter, their dives are also deeper and longer compared to summer. Hooded seals are mostly alone, except during breeding and molting seasons. During these times, they do not eat for a period. Hooded seals gather near the Denmark Strait in July during their molting season to mate. Compared to other seals, hooded seals are less social and often act aggressively and protect their territory. They show aggression by inflating the "hood," a feature described in the "Nasal Cavity" section. Hooded seals travel long distances and spend most of the year alone, except during mating season.
Nasal cavity
The hooded seal has a special, stretchy nasal cavity on the top of its head, called the hood. Only male seals have this feature, which they start to grow around the age of four. When a seal dives underwater, the hood begins to inflate as it takes its first breath. While swimming, the hood repeatedly inflates and deflates. This action helps the seal make sounds to signal danger and scare away other animals when competing for food or shelter. It also helps the seal show its health and dominance to other males and females it is trying to attract. Male seals that are ready to mate have a pink, balloon-like membrane that comes out of the left nostril. This membrane can create different sounds and calls when shaken, depending on whether the seal is underwater or on land. Most of these sounds are used for communication during normal activities (about 79%), while about 12% are used to attract mates.
Breeding and life cycle
Hooded seals have four main areas where they breed: the Gulf of St. Lawrence; the "Front" east of Newfoundland; Davis Strait, which is between Greenland and northern Canada; and the West Ice near Jan Mayen. Male hooded seals often mate with more than one female during a single mating season, which suggests they are polygynous. Some males stay with one female for a long time, while others move around more and mate with several females for shorter periods, helping to increase the number of offspring in the population. Most males become sexually mature by the age of 5.
Across all regions, hooded seals give birth in late March and early April. They shed their old fur, or molt, from June to August. The four known groups of hooded seals are usually divided into two main populations: a Northeast (NE) Atlantic population and a Northwest (NW) Atlantic population. About 90% of the NW population gives birth on the "Front." The NE population, which breeds near Jan Mayen, typically moves into the ocean after giving birth in March. From April to June, after the breeding season, this species travels long distances to find food before gathering again. Some seals return to the same ice area in July to molt, but most molt farther north. After molting, the seals spread out again to feed during late summer and autumn, then return to their breeding areas in late winter.
Offspring
Pups are about 1 meter (3 feet 3 inches) long when they are born and weigh around 24 kilograms (53 pounds). They are born on the ice between mid-March and early April. They have a thick layer of blubber and have already shed their prenatal coat. They have a slate blue-grey coat, which is why they are sometimes called "blueback." Their bellies are pale cream, and they shed this coat after about 14 months. The pup is nursed for about 4 days, the shortest lactation period among all mammals. During this time, the pup doubles in size, gaining about 7 kilograms (15 pounds) each day. This rapid growth is possible because the milk they drink has 60% fat. Female pups reach maturity between the ages of 3 and 6, while male pups mature between the ages of 5 and 7.
Early development
Scientists have found that the way seal pups grow their muscles is different from how adult seals grow theirs. This happens because pups need to work and find food underwater in ways that are different from adults. Studies show that the heart’s blood flow gives enough oxygen to help seal pups use energy during dives, which helps them deal with not having enough oxygen. At birth and during the time they are learning to live on their own, the heart muscles are more developed than the muscles used for movement. However, neither the heart nor the movement muscles are fully grown by the end of the weaning period. Seal pups are born with enough hemoglobin (a protein in blood that carries oxygen) but have only about 25 to 30 percent of the myoglobin (a protein in muscles that stores oxygen) found in adult seals. These findings show that pup muscles are not as good as adult muscles at making energy through either aerobic or anaerobic processes during dives. This is because adult seals rely on large oxygen stores in their blood and muscles to stay underwater for long periods. This might explain why seal pups have a short weaning period, as diving is important for their survival.
Hooded seals lose their lanugo fur (a type of fur) while still in the womb and are born with a thin layer of fur that is not as good at keeping them warm as the fur of some other seal species, like leopard seals. Research shows that after birth, newborn hooded seals grow blubber (a thick layer of fat) quickly, which helps them stay warm and makes up for their thin fur. This process of losing fur before birth and relying on blubber may be helpful because water cannot pass through blubber as easily as it can through fur. This allows blubber to keep seals warm in both water and air.
Threats and conservation practices
Before the 1940s, adult hooded seals were mainly hunted for their leather and oil. Today, the biggest threats to hooded seals are hunting, which includes hunting for food, and bycatch. Seal strandings, which happen when seals are washed ashore, are not a major threat but are studied closely. Seal pups are hunted for their blue and black fur, and many mothers are killed while trying to protect their young. Hunting occurs mainly in Greenland, Canada, Russia, and Norway. Northwest Atlantic hooded seal populations are stable or growing, but northeast Atlantic populations have decreased by 85–90% over the past 60 years.
Scientists once thought sonar caused large numbers of hooded seals to strand. After testing sonar on seals in captivity, with frequencies from 1 to 7 kHz, it was found to have little effect. Some tests showed changes in diving or swimming behavior, but these differences only appeared during the first exposure.
Conservation efforts, including international cooperation and the creation of the Northwest Atlantic Fisheries Organization (NAFO), helped hooded seal populations grow. Now, a permit is required to hunt hooded seals in international waters, and each permit has a set limit. The total number of hooded seals that can be hunted each year is 10,000.
The hooded seal is protected by the Marine Mammal Protection Act of 1972.
- A mother hooded seal with her pup.
Predators of hooded seals include killer whales, polar bears, and Greenland sharks.